1. Field of the Invention
The present invention relates to the preparation of coated catalysts based on the molybdates of bismuth and iron and doped with phosphorus and potassium values.
This invention especially relates to the preparation of such coated catalysts via the production of a catalytically active intermediate composition, next calcining and grinding this intermediate and then coating particles of a solid and inert support substrate, the external surface of which is rough, with said ground composition or a mixture comprised thereof, followed by the calcination of the particulates thus coated.
2. Description of the Prior Art
French Patent No. 2,047,199, equivalent to U.S. Pat. No. 3,959,384, describes oxidation catalysts corresponding to the general formula: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d L.sub.e M.sub.n Mo.sub.f O.sub.g
in which
L is particularly phosphorus, PA0 M is particularly potassium, PA0 a and b are numbers ranging from 0 to 15 and the sum (a+b) ranges from 2 to 15, PA0 c is a number ranging from 0.5 to 7, PA0 d is a number ranging from 0.1 to 4, PA0 e is a number ranging from 0 to 4, PA0 f has a value of 12, PA0 g is a number ranging from 35 to 85, and PA0 n is a number ranging from 0.01 to 0.5
These catalysts are prepared by formulating a suspension in aqueous medium from various precursors of the elementary constituents of the catalyst, by adding a support (such as a silica gel) to said suspension which is essentially a paste, and by heating this to dryness to provide a cake which is then treated at elevated temperature in the presence of air.
The catalysts are employed in the form of particles or of tablets.
These catalysts, both in bulk form and diluted, are effective, but they present difficulties over the course of an oxidation process on an industrial scale. Indeed, in a fixed bed, locally elevated temperatures may arise to initiate an undesirable violence of the reaction.
French Patent No. 2,202,729, equivalent to U.S. Pat. No. 4,077,912, describes that it is advantageous to employ catalysts for the oxidation of propylene to acrolein which are prepared by coating, namely, formed of a catalytically active layer of analogous composition, but deposited onto the external surface of an inert support of at least 20 microns in diameter, instead of diluting it with a support introduced with the metallic salts. It is then possible to better control the evolution of the heat of reaction in fixed beds processes.
Nevertheless, this particular technique for producing the catalyst requires a significant portion thereof to be constituted by the inert support (66% by weight of the finished catalyst, according to the sole example of this '729 patent). The fraction reserved for the active phase in comparison with the former simply diluted catalysts is decreased, which results in a very disadvantageous decrease in the activity of the catalysts.
This may manifest itself industrially in the obligation either to use larger reactors to preserve the production capacity and identical operating conditions, or to increase the reaction temperature to preserve the production capacity and the size of the reactor. In the first instance, the major disadvantage is economic. In the second instance, two disadvantages are presented: the selectivity for acrolein will diminish and the activity of the catalyst will decrease more rapidly over the course of time.
U.S. Pat. No. 4,298,763 describes, for the oxidation of propylene into acrolein, a calcined catalytic composition (active phase) corresponding to the general formula: EQU Mo.sub.12 Bi.sub.0.1-4 Fe.sub.0.5-6 M2.sub.b M3.sub.c M4.sub.d M5.sub.e O.sub.x
in which M2 is nickel and/or cobalt, b is a number ranging from 2 to 12, M3 is particularly K, c is a number ranging from 0.01 to 0.1 and preferably from 0.03 to 0.09, M4 is P, d is a number ranging from 0 to 1 and preferably from 0.01 to 0.02, M5 is In and/or Na, e is a number ranging from 0 to 0.5 and preferably from 0.01 to 0.02, and x is the number of atoms of oxygen required to satisfy the valencies of the other constituents.
This active phase is deposited as a layer of thickness 150 to 1500 .mu.m and of surface area less than 15 m.sup.2 /g.
The deposition of the layer of calcined and pulverulent catalytic material, the dimension of the particles ranging from 0.1 to 300 .mu.m, is carried out in moist medium, the support particles being vigorously stirred and controlled operating conditions moreover being required.
The layer coating the central supporting core constituted at least 50% of the weight of the support, namely, at least 33% by weight of the finished catalyst and at most 250% of the weight of the support, namely, at most 71.4% by weight of the finished catalyst.
Prior to being used for the oxidation of olefins, the coated catalyst is dried and, if necessary, calcined at a temperature of 400.degree. to 700.degree. C.
It will be seen from this '763 patent that the totality of the constituents of the catalytic composition coating the particular support is present prior to the calcination and the grinding of such composition.
Example 1 of the patent describes a composition containing 0.06 atoms of phosphorus and 0.06 atoms of potassium per 12 atoms of molybdenum, the former two elements being introduced into the aqueous solutions in the form of KOH and phosphoric acid before precipitation, and also atomization of the entirety of the suspension. The atomized precursor is then remixed with another aqueous potash solution before extrusion and then drying and calcination.
The dopants are therefore introduced into the precursor of the active phase before such active phase forms during calcination. This technique for the introduction of the dopants values presents the following major disadvantages:
(1) a greater complexity because it is necessary to introduce the potash in two different operations, which necessitates two weighings and therefore gives rise to two risks of error;
(2) a significant loss of phosphorus dopants within the particles of precursors, even though the desirable function of the dopants is exerted on the surface of the particles of the active phase, whereat the catalytic reactions occur;
(3) a poor control of the microscopic distribution of the potassium added after the atomization, by mixing. Such a precursor, merely dried by atomization, generally includes sites of absorption of cations of sufficient strength to obstruct the homogeneous distribution of all of the potassium on all of the particles of solids. A poor control of the atomic ratio of phosphorus to potassium on the particle scale results, an atomic ratio whose value significantly affects the activity of the finished catalyst.
U.S. Pat. No. 4,621,072 particularly relates to a process for the preparation of coated catalysts which are resistant to abrasion, comprising an inert support having a rough surface and particle dimensions of 0.5 to 6 mm and a layer of catalytically active material coated onto the support and fixed thereto.
This '072 patent describes in detail the difficulties encountered in the preparation of coated catalysts in order to obtain a coating layer having mechanical properties sufficient that the catalysts can be used on an industrial scale in fixed bed reactors. The patent also proposes various measures to overcome these disadvantages and recommends, inter alia, using a suspension of a precursor of the catalytically active material also containing a binder and, if necessary, a porogenic agent. In this instance too, the precursor contains all of the constituents required for the production of a catalytically active material via a final specific thermal calcination treatment.
It will also be seen that particular care is required in the coating process to attain a good mechanical resistance despite a significant proportion by weight of active phase in the finished catalyst.
In this instance also the aforesaid technique for the introduction of the potassium and/or phosphorus dopants presents the above disadvantages.
In addition, in this process, as in all of those discussed above, the suitable soluble salts used in the preparation of the catalytically active phase are the metallic nitrates, ammonium molybdate, potassium nitrate and phosphoric acid. Ammonium nitrate will therefore be formed during this preparation and will decompose thermally at approximately 230.degree. C. during the subsequent calcination.
It is apparent that this thermal decomposition of ammonium nitrate poses serious practical problems. Indeed, one skilled in this art is well cognizant that ammonium nitrate is an explosive compound and, in this respect, is dangerous to handle at the industrial level.
French Patent No. 2,481,146, equivalent to U.S. Pat. No. 4,382,880, moreover describes a process for the preparation of catalysts based on the oxides of molybdenum and/or tungsten and the oxides of other metals. This process permits the thermal decomposition of ammonium nitrate to be avoided. It is characterized in that, in a first stage, to a first aqueous solution containing the ammonium, molybdenum and tungsten salts, a second solution containing the metallic salts is added. In a second stage, ammonia is added to the mixture obtained until a pH greater than the pH of the beginning first aqueous solution is filtered to provide the cake which will then be calcined to produce the active phase.
This procedure enables the ammonium nitrate, which is soluble in the mother liquors, to be easily separated, on the one hand, and the precursor of the active phase, in the form of a filter cake, on the other. It is applicable to formulations based on molybdates of bismuth and of iron described in the above patents with the exception that, since one skilled in this art is well aware that potassium nitrate and phosphoric acid are soluble under the conditions employed, these elements will be entrained in the mother liquors. It is therefore not possible in this manner to introduce a specific amount of phosphorus and of potassium into the active phase in a controlled manner.